In recent years, digital still cameras (hereafter referred to simply as digital cameras) and video cameras have become remarkably popular. In these cameras, an optical image is converted into electrical signals using an image pickup device, such as a CCD (charge coupled device), and the signals are recorded as image data.
As a zoom lens used for these cameras, various lenses have been previously proposed. When the desired zoom ratio is approximately three, a zoom lens that has a two-group or three-group construction is widely used. In order to obtain a lightweight lens and high-speed autofocus, an inner-focus format or a rear-focus format is preferred. In implementing these features, a three-group construction is used instead of a two-group construction. The applicant of the present invention has proposed zoom lenses with such construction in Japanese Laid-Open Patent Applications H10-293253 and 2000-284177.
The three-group zoom lens of Japanese Laid-Open Patent Application H10-293253 includes, in order from the object side, lens groups of negative, positive, and positive refractive powers with zooming performed by movement of only the object side and middle lens groups, while focusing is performed mainly by movement of the image-side lens group. The zoom lens satisfies two conditions for obtaining a desired distance along an optical axis between an image plane and the exit pupil in order to enable an appropriate zooming ratio, a light flux that strikes the image plane in a substantially telecentric relationship, and imaging without substantial vignetting.
The three-group zoom lens of Japanese Laid-Open Patent Application 2000-284177 performs zooming by moving all three lens groups with the middle and image-side lens groups moving together toward the object side, when infinity focus is maintained, from the wide-angle end to the telephoto end so that they become closer to the object-side lens group. In this zoom lens, the distance along the optical axis between the image plane and the exit pupil remains appropriate with the rear-focus format.
The three-group zoom lens of Japanese Laid-Open Patent Application 2002-196240 includes, in order from the object side, lens groups of negative, positive, and positive refractive powers. The middle lens group includes, in order from the object side, a lens element having positive refractive power that is a lens component, a lens component that includes a lens element that has positive refractive power cemented to a lens element having negative refractive power, and another lens component that is a lens element. When zooming from the wide-angle end to the telephoto end, all three lens groups move with the middle lens group becoming closer to the object-side lens group and farther from the image-side lens group. The image-side lens group acts as a variator to adjust the position of the image plane along the optical axis due to zooming.
In recent years, with digital cameras generally, a lens with an f-number FNO of approximately 2.8 and high image resolution has been desired. Additionally, further miniaturization of the digital cameras generally is also desired, which may be accomplished by miniaturizing the zoom lens that affects the overall size of the digital camera. Furthermore, CCD's for use in digital cameras where the total number of pixels is beyond five million have been developed, and because the trend is that the number of pixels will increase in the future, a much higher image resolution is required for zoom lenses used in digital cameras generally.
However, with the zoom lenses disclosed in the above-mentioned Japanese Laid-Open Patent Applications H10-293253 and 2000-284177, the f-number at the wide-angle end is limited to about 3.2 or higher, so that brighter (smaller f-number) lenses may be required for various uses. Also, in the zoom lens disclosed in Japanese Laid-Open Patent Application H10-293253, the object-side lens group includes four lens elements, so that further improvement in compactness and a lighter weight of the zoom lens is required. Additionally, in the zoom lens disclosed in Japanese Laid-Open Patent Application 2000-284177, the middle and image-side lens groups move as a unit together during zooming, and that tends to make the entire length of the zoom lens longer at the wide-angle end, making it more difficult to obtain a desired zoom ratio while maintaining overall compactness of the zoom lens.
Although the zoom lens disclosed in Japanese Laid-Open Patent Application 2002-196240 provides a bright image with an f-number of approximately 2.8, the image-side lens group, as well as the object-side and middle lens groups, move during zooming. With this construction, the refractive power of the image-side lens group becomes relatively large. As a result, the eccentric sensitivity (the degree of optical performance deterioration due to the eccentricity of alignment of the lens elements) becomes great. In particular, many recently made digital cameras are constructed for greater miniaturization so that the zoom lens fully retracts or collapses to a very short overall length when not in use. In zoom lenses like the one disclosed in Japanese Laid-Open Patent Application 2002-196240, the object-side and middle lens groups are mechanically secured by one cam cylinder. The image-side lens group is independently secured and thus performance deterioration due to the eccentricity of the image-side lens group easily becomes a problem. On the other hand, if the object-side and image-side lens groups were to be secured by one cam cylinder, eccentricity of the image-side lens group relative to the other two lens groups might be restrained, but it becomes difficult for the zoom lens to be made collapsible to a short overall length.
In addition, in the zoom lenses disclosed in all three of the above-mentioned Japanese applications, although substantial correction of aberrations is achieved, higher correction is required in response to the recent demand for higher image resolution.